Since a mobile radiotelephone does not have an accurate enough frequency reference internally, it must make fine frequency adjustments to achieve the required frequency synchronization to a base station frequency. In a time domain multiple access (TDMA) system, multiple logical channels are transmitted on the same frequency, but separated in time, FIG. 2. To communicate with a base station, the mobile radiotelephone must also find the boundaries of these time slots, called time slot synchronization.
The different base stations in a cellular radiotelephone system maintain very accurate frequency references, but utilize different transmission frequencies, and possibly different time slot alignments. When a mobile radiotelephone is handed off from one cell to another in a cellular radiotelephone system, the mobile may need some minor frequency adjustment, as well as complete time slot synchronization to communicate with the new base station.
To accomplish this in a digital cellular radiotelephone system, the radiotelephone first finds a frequency correction channel (FCCH), which is part of the broadcast control channel (BCCH). FIG. 2 illustrates the FCCH slots (201) and other data control channels that make up the multiframe TDMA structure of the BCCH. This format is described in greater detail in the digital cellular standard specification GSM Recommendation 5.02, Version 3.3.1, Oct. 13, 1989.
The base band signal of the FCCH is a frequency correction burst (FCB), a pure tone (sine wave) at 67.5 kHz, consisting of 148 samples, sent periodically and it always occurs in time slot zero of the data stream. The offset between the carrier frequencies of the base station and the mobile radiotelephone is translated to the base band as a deviation from 67.5 kHz. The boundaries of the FCB delineate the time slots of the TDMA structure. From the FCCH detected, the mobile radiotelephone synchronizes its local oscillator frequency and time slot boundaries with those of the base station using the frequency correction burst in the FCCH time slot.
Since the burst is relatively short, the mobile must find it in the data stream and synchronize with it in this short period. There is a resulting need for a process that can detect the presence and boundaries of the FCB very rapidly, and estimate the frequency offset very accurately, even when signals are received in the presence of noise.